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The Fatigue Severity Scale (FSS) is meant to evaluate the impact of fatigue on a person’s life. (It doesn’t really measure “severity” but it is quite useful.) The FSS was developed by Lauren B. Krupps and colleagues in the late 1980s as a tool to assess fatigue in patientswith multiple sclerosis and systemic lupus erythematosus. The scale is sometimes referred to as the Krupp’s Fatigue Severity Scale in the medical literature. An application of a Rasch model commonly used in psychometrics, the FSS is used in quality of life assessment for many therapies, policies, etc. for chronic diseases and sometimes in sleep research and clinical sleep medicine. The scale is also of use in evaluating patients with major depression.
The FSS questionnaire is comprised of nine statements inquiring about the examinee’s sleep habits over the preceding week. Ratings are on a 7-point Likert scale, where higher scores indicate how strongly the patient agrees with the nine statements.
A score of 36 and above (out of a maximum of 63) indicates the presence of significant fatigue but typical scores are much lower. The researchers behind a Norwegian study of a general population concluded a score of 45 would be better for indicating significant fatigue. It has been suggested that the FSS is most effective as a measure of disability-related fatigue, that is, fatigue arising from a chronic, debilitating illness. The FSS has become a major tool in studying fatigue in multiple sclerosis—over half of all studies on the topic have used the scale since its inception.
Studies to validate the FSS have shown the scale is internally
consistent and can detect changes in fatigue over time. The research community and some of the clinical community have accepted the FSS as a simple and reliable measure of fatigue. It is not used as much as the MSLT or Epworth scale in the diagnosis of sleep disorders and in trials of therapies, but in that the FSS scores correlate with external observations of fatigue, it has its place.